Carbon nanotubes (CNTs) are considered attractive materials for use in energy storage devices because of the individual nanotube properties of high strength and electrical conductivity. Carbon nanotubes have been recognized to be potentially useful in lithium ion batteries because of the lithium ion intercalation with the graphene layers, as described in U.S. Pat. No. 7,060,390. The challenge with current widespread use of carbon nanotubes, particularly with single walled and doubled walled carbon nanotubes is the absence of a robust, efficient and innocuous method to completely debundle nanotube aggregates into an individually dispersed state (i.e., exfoliated state). Thus, the previous use of carbon nanotubes in energy storage devices have been limited in performance by not being able to fully access the active surface area. Likewise the CNTs are difficult to obtain of high purity (>about 96 percent by weight) through removal of their catalytic residues and non-tubular carbon structures arising from their synthesis.
The obstacle to exfoliating CNTs arises because immediately following their synthesis the tubes readily assemble into parallel configurations leading to what is commonly referred to as bundles or ropes. As a consequence formidable van der Waals binding energies of about 20 kbT for every nanometer of tube overlap result, and hence, formation of aggregates that are very difficult to separate completely occurs. To overcome the van der Waals forces various approaches have been employed, such as tube chemical functionalization, surfactants and the like. These approaches have only been successful at producing exfoliated nanotubes of higher yields after severe degradation of the initial tube length. Carbon nanotubes of much reduced length suffer from poorer strength and conductance and thus limit their full performance in energy storage or collection devices.
Aligned carbon nanotubes still have considerable van der Waals associations which cause local clumping of the carbon nanotubes and hence reduced active surface area. Also, the challenge with aligned carbon nanotubes composites is that cracking in the tube direction can occur more easily than randomly oriented carbon nanotube composites. Additionally, the cost associated with specialty techniques for growing the carbon nanotubes in vertical arrays and their handling in making commercial electrodes is thought to be prohibitively high.
Lithium, Li, ion batteries are receiving considerable attention in applications, ranging from portable electronics to electric vehicles, due to their superior energy density over other rechargeable battery technologies. However, demands for lighter, thinner, and higher capacity lithium ion batteries has necessitated a concerted development of both improved electrodes and electrolytes to extend battery capacity, cycle life, and charge-discharge rates while maintaining the highest degree of safety available.
Li-ion batteries for vehicles typically require three times higher energy densities than available at present to meet the volume/weight requirements and to reduce the number of cells in the battery and system cost. Li batteries are not intrinsically tolerant to abusive conditions such as a short circuit (including an internal short circuit), overcharge, over-discharge, crush, or exposure to fire and/or other high temperature environments. The use of Li chemistry in these larger (energy) batteries increases the urgency to address these issues. The ability to attain a 15 year life, or 300,000 HEV cycles, or 5,000 EV cycles are unproven in conventional Li ion batteries and are anticipated to be difficult due to undesirable volume expansions/failure at electrodes and side-reactions of Li with the electrolyte at voltages greater than about four volts.
Batteries generally include a cathode, an anode and an electrolyte. Commercially, the most popular material for the anode of a Li-ion battery is graphite. The cathode is generally one of three materials: a layered oxide, such as lithium cobalt oxide, one based on a polyanion, such as lithium iron phosphate, or a spinel, such as lithium manganese oxide. The common lithium ion battery as commercialized by Sony uses an inorganic compound, LiCoO2 as the cathode material and graphite at the anode. The LiCoO2 has a rhombohedral structure where Li and Co cations fill alternating layers of edge-sharing octahedral sites in a close packed oxygen array. During charging, lithium is de-intercalated from the cathode layers, transported across the separator membrane in an electrolyte medium, and then intercalated into the carbon anode. In the discharge process, the lithium ions are de-intercalated from the anode and intercalated again to the empty octahedral site between layers in the cathode. Depending on the choice of material for the anode, cathode, and electrolyte the voltage, capacity, life, and safety of a lithium ion battery can change dramatically. A challenge for batteries in general is to manage the heat generated at the anode during discharge. The heat causes degradation of the electrolyte and hence reduced energy capacity over time.
The specific energy density (per weight or per volume) is related to both the working voltage and the reversible capacity. The working voltage depends on the potential of the redox process and the reversible capacity is restricted by the reversible amount of lithium intercalation. The available redox pair should locate in a higher and suitable potential range and the structure of material should be stable in wide composition range in order to obtain a high capacity.
The electrochemical lithium insertion/extraction reactions involve both lithium Ions diffusion in the lattice and charge transfer process on the particle surface. Thus, the electrode's conductivity includes lithium ion conductivity in active material bulk and electronic conductivity of electrode. Higher electronic conductivity is helpful to keep the inner resistance low and gives an excellent power density. Routes to overcome this deficiency include reduction of particle size and increase in electronic conductivity by coating of conducting agent such as carbon, as described in WO 2009/133807.
Silicon nanowires would appear to have high potential for future battery applications because of their inherent storage capacity of 4200 mAh g−1. However, silicon expands over 300% upon Li+ insertion, leading to severe problems of cracking on charge/discharge cycling. US 2008/0280207 describes an anode structure consisting of a silicon layer (not nanowires) around a parallel array of carbon nanotubes as being beneficial for improved capacity. The silicon layer is deposited by using chemical vapor deposition of SiH4. The carbon nanotubes are also not exfoliated.
Conducting or high dielectric polymers such polyaniline, polypyrrole and polyvinylidene fluoride are often selected for binders of electro active particles.
The most popular electrolytes are the liquid-type ones where carbonates or esters of simple alcohol and glycol are frequently used as solvents which contain LiPF6 as an electrolyte. Solvents typically are a mixed solution of ethylene carbonate (EC) of high dielectric constant and methyl ethyl carbonate (MEC) of low viscosity. Sometimes a combination of □-butyrolactone and LiBF4 is utilized. Propylene carbonate is an excellent solvent, but it decomposes rapidly on the surface of graphite. If there is a short circuit, very significant heat buildup (>200° C.) can occur and ignite these types of electrolytes.
Recently, polymer electrolytes have attracted much attention because they enable freedom from electrolyte leakage and can make a thin battery. Solid-state electrolytes and some polymer electrolytes need no separator. Many kinds of polymer electrolytes have been proposed, but only a few are utilized in practical batteries. Polysiloxane is one of recent interest. Many solid polyelectrolyte types are not a true solid polymer, but a polymer gel containing liquid electrolyte as a plasticizer.
The separator has two primary functions: one is to avoid the direct contact between the anode and cathode, while it allows a free mass transfer of the electrolyte, and the other is a shutter action to stop the mass transfer in the case of accidental heat generation. The separator film melts resulting in pore closure. Biaxially orientated polyolefin film is commonly used to obtain a high porosity film.
A composite anode material made of silicon/graphite/multi-walled carbon nanotubes (MWNTs) for Li-ion batteries has been prepared by ball milling. This composite anode material showed a discharge capacity of 2274 milliamp-hours per gram (mAh/g) in the first cycle, and after 20 charge-discharge cycles, a reversible capacity of 584 mAh/g was retained, higher than 218 mAh/g for silicon/graphite composite. However, the silicon particles appeared to be on a scale of about a micrometer in diameter and were irregularly distributed. Further, no attachment of the particles to the MWNT was apparent.
Vertically-aligned multi-walled carbon nanotube (VAMWNT) electrodes grown on substrates such as aluminum or silicon have been investigated. The current state-of the-art of lithium-ion batteries utilizes graphite as a negative electrode with a maximum theoretical specific capacity of 372 mAh/g and a practical specific capacity ranging from 150-370 mAh/g, which were aligned in the direction of current flow. By aligning the nanotubes in this manner, increased access and interfacial dynamics between lithium-ions and the interstitial spaces of the MWNTs as well as the internal and external surfaces of the MWNTs were thought possible. These electrodes were able to produce a stable and reversible capacity of 650 mAh/g. As mentioned previously, fully aligned carbon nanotubes as made can still associate to form bundles and cracking is more likely along the tube lengths.
Aligned carbon nanotube coaxial nanowires have also been prepared by electrochemically depositing a concentric layer of an appropriate conducting polymer or titanium dioxide, TiO2, coating onto the individual aligned carbon nanotubes. These aligned carbon nanotube coaxial nanowires were demonstrated in the laboratory to possess unique electron transfer properties and speculated to have potential significance for a wide range of device applications, including batteries and supercapacitors.
Mats of carbon nanotubes and carbon particles have been utilized as conductive systems to replace metal foils. Impregnation of xerogels of V2O5 composite electrode gave a reversibility specific capacity of 160 mAh.g−1 at a constant discharge/charge current of 95 pk mA.g−1 between 4 and 2V versus Li/Li+ Simple impregnation methods do not control the spatial distribution of the particles to prevent local charge density fluctuations and stable structure over time. Control of the distribution of the nanoscale particles or layers by attachment is believed to be beneficial to maintain the high crystal surface area to volume ratio.